The present invention relates to an optical amplifier apparatus that is suited, in particular, for amplifying at a high amplification factor a laser beam emitted from a laser source or from an oscillator.
The achievable laser output power, in particular of solid-state lasers, is generally limited with regard to high beam quality by the thermal lens effect of the amplifying medium. An oscillator-amplifier apparatus has been in use to date to achieve high laser performance accompanied by high beam quality. The oscillator is arranged in such a way that it emits a laser beam of high quality at relatively low power. Subsequently, the laser beam, which is emitted by the oscillator, is irradiated into the amplifier (connected in series) and amplified, resulting in high beam power while maintaining the beam quality.
An apparatus of this type, including an amplifier connected in series, is shown e.g. in FIG. 6, wherein the reference symbol 1 denotes an optical amplifier, reference symbol 13 denotes an oscillator and reference symbol 14 denotes the optical components that serve to realize the optical imaging.
The apparatus represented in FIG. 6 is a laser apparatus in which both the oscillator and the amplifier have a rod-shaped solid-state medium. Based on the available amplification, which is limited ultimately because of increased spontaneous emission and parasitic oscillation, the typical amplification factor for an amplifier of this kind is approximately 1.2 to 3 per run.
In weak oscillators, this amplification factor is often too minimal to achieve an efficient utilization of the amplification medium.
One possibility for solving this problem is a regenerative amplifier apparatus where the amplification medium is integrated in a resonator with a Pockels cell and a polarizer (regenerative resonator). The laser beam that is to be amplified is injected in the regenerative resonator through the Pockels cell and the polarizer. After several runs through the amplifier medium, the laser beam is amplified several times, and in the end it is coupled out of the resonator via the Pockels cell and the polarizer. Even though this method produces an efficient amplification, the described realization is very complex and is applicable only to a limited degree for laser pulses that are shorter than several 10 ps.
To further increase the laser performance several amplification steps can be used. Multi-step amplifier apparatuses of this kind always come at a high cost. Moreover, they are very voluminous, need much space and are not very reliable.
To improve the amplification factor, the U.S. Pat. No. 4,703,491 discloses a laser apparatus with a partially permeable feed-out mirror and a fully reflective folding mirror on the one side of the active laser medium and another fully reflective folding mirror on the other side of the active laser medium. The optical system is arranged in such a way that a laser beam passes through an active laser medium several times resulting in a long, effective resonator length. In this apparatus, the beam returns on to itself after multiple folding or multi-pass.
Furthermore, German patent document DE-A-196 09 851 describes a micro-strip laser with an almost fully reflective end mirror and a partially reflecting feed-out mirror that are arranged, respectively, on different sides of the active laser medium, and two folding mirrors that are inclined in the direction of the face sides of the resonator on both sides of the laser medium. This apparatus realizes a multi-pass resonator. As shown in particular in FIG. 3, one of the folding mirrors has a mirror surface with a concave curvature, which expands the cross-section of the laser beam in one pass-through direction. In the reverse pass-through direction, however, the cross-section of the laser beam is once again reduced. This leads to increased stress being placed on the optical components within the resonator. In fact, when passing from the feed-out mirror to the fully reflective end mirror, the power density drastically increases because the cross-section of the beam is becoming smaller. This not beneficial for the effective utilization of the amplifier medium and in terms of the stress on the optical components.
It is therefore an object of the present invention to provide an optical amplifier apparatus that eliminates the disadvantages of the state of the art.
This and other objects and advantages are achieved by the optical amplifier according to the invention, which includes an amplifier medium that has approximately a rectangular cross-section, with a long edge and a short edge, and at least two highly reflective mirrors. The amplifying medium is arranged between the latter mirrors, with the long edge or short edge of the cross-section disposed along the x-axis or the y-axis, and the z-axis being the optical axis. (The x-, y- and z-axes constitute a rectangular system of coordinates.) The mirrors are designed and arranged in such a manner that a beam injected by an oscillator into the xz-plane, which is to be amplified, passes through the amplifying medium several times and is amplified, while the dimension of the beam, that is to be amplified, expands in the x-direction after each pass-through.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.